In recent years, satellite relays have come into common usage in allowing communication between cities separated by large distances. For example, such satellite relays are now used in a large number of cases for the communication of voice, teletypewriter (TTY), and television signals from one city to another. In this process, a terminal station such as a communication switching center (e.g. a telephone central office) is provided in each city. This communication switching center receives signals from and transmits signals to individual subscribers to provide baseband information to the subscribers.
In a simple system for transmitting from a first city to a second city using the well-known single channel per carrier approach (SCPC), the baseband information at the communication switching center is converted to a composite multi-carrier intermediate frequency signal and then to a composite multi-carrier radio frequency signal. A separate carrier is provided for each baseband signal, and these separate carriers are added to produce the composite multi-carrier radio frequency signal. This composite radio frequency signal is transmitted as a satellite up link channel signal typically having a 36 MHz bandwidth portion of the frequency band between 5.9 and 6.4 GHz. Each such up link channel with its 36 MHz bandwidth has between 400 to 800 individual carriers separated by 45 kHz for individual channels having a .+-.12 kHz bandwidth.
The satellite relay takes this up link multi-carrier signal and converts it to another composite multi-carrier satellite down link signal which also has a 36 MHz bandwidth but with a different frequency band than the up link signal (for example, typically in the frequency band between 3.7 and 4.2 GHz). The down link signal is received at the communication switching center at the second city where it is converted to a composite multi-carrier intermediate frequency signal and subsequently to baseband signals. These baseband signals are then used for transmission to the individual subscribers at the second city. Of course, the systems are generally designed for duplex operation for communication back and forth between the two cities.
Although the basic arrangement described above works well in areas where space is not at a premium, it has a serious drawback in most practical instances. Specifically, a very large antenna, for example often on the order of 8 to 16 meters, or even larger, is necessary for respectively receiving and transmitting the satellite down link and up link signals. Such large antennas, along with all their associated hardware, are often quite objectionable and impractical in cities.
Accordingly, to eliminate the need for such large antenna systems located in a metropolitan area, ground relay stations are generally provided at a location remote from the city. The ground relay station receives the down link satellite signal and transmits the up link satellite signal. The large antenna and hardware necessary for directly handling such satellite signals are located at the ground relay, well outside of the actual metropolitan area. The ground relay serves to relay the information between the satellite and the communication switching center in the city over a microwave relay link. Typically, this microwave relay link is at a frequency different than the respective up link and down link satellite signals. The size of the antenna necessary at the communication switching center to communicate with the ground station is much smaller than that necessary for direct satellite communication.
FIG. 1 provides an overall block diagram view of a satellite relay system for communicating between two distant communication switching centers 10 and 12 using a satellite relay 14 and two ground relays 16 and 18. Both ground relays 16 and 18 are typically located at a distance of several miles from the communication switching centers. As an example of signal transmission, data from the communication switching center 10 is relayed to the other communication switching center 12 through the ground relay 16, the satellite relay 14, and the other ground relay 18.
In prior systems of this type, the ground relay stations 16 and 18 processed the entire received satellite down link signal to recover all of the original baseband signals therefrom. Local information which is needed at the ground relay itself can be stripped off for direct usage in the vicinity of the ground relay. The remaining baseband signals are again processed to form a single composite FDM-FM radio frequency signal for transmission to the communication switching center 10 or 12. This newly produced FDM-FM composite signal normally is different in both frequency band and bandwidth in comparison to the up link and down link signals. Upon receipt of this new FDM-FM signal, the communication switching center 10 or 12 reduces this composite FDM-FM signal to baseband signals for transmission to individual subscribers. The up link operation is the reverse of this down link operation as just described.
Although this ground relay arrangement does allow removal of the direct satellite reception and transmission equipment from metropolitan areas, it also has a number of serious drawbacks. In the first place, because the ground relay reduces the entire down link and up link satellite signals to baseband before conversion to a composite multi-carrier radio frequency signal, a separate receiver is required for each baseband channel. In a typical modern system, up to 800 one-way baseband channels are provided in each satellite channel. Most satellites now provide either 12 or 24 satellite channels. Therefore, the ground relay has to provide up to 800 receiver-transmitter units for conversion of the baseband signals for each satellite channel. Thus, in a 24-satellite channel system, 19,200 receiver-transmitter units are required at the ground relay due to this reduction to baseband arrangement.
In addition to the large number of receiver-transmitter units necessary for baseband processing, the ground relay station also requires equipment for multiplexing the baseband signal and FDM-FM modulation and demodulation equipment for putting the signals into a form suitable for transmission as a composite multi-carrier radio frequency signal. Such equipment can be extremely costly. Although this cost can be justified to some extent in areas where a very high line usage occurs, it has a very low cost effectiveness in situations of low line usage.
Although, as mentioned above, it is sometimes desirable to strip off some signals for use at the ground relay, it is generally not necessary that all of the signals be reduced to baseband. The reason is that these signals are not actually needed in baseband form until they reach the communication switching terminal. In fact, in addition to such reduction of all signals to baseband being unnecessary, it also creates dialing interface problems. Further, it makes voice communication non-secure at the ground relay station because all voice conversations can be listened to at the ground relay station.